Easy. just import one hemi into PW and a second Hemi that has it's round image coloured white. Ensure that the white circle image is the first image and ur to be viewed hemi is the second when importing to PW. Stich and the result will be ur half hemi streched with a white frame. Better still, as the white image was the first one in PW it means that the output ensures that the hemi u want to display actually shows up in the thumbnail as the image to be seen instead of being cut in half. Also thgis will enable u to constrain the amount of movement left, right, up and down.
An example of this u'll see following this link where I've added a number of single hemis in the tours. Just choose ur conection speed when u arrive to the splash page. here
If I only had an hour to chop down a tree... I'd spend 45 mins sharpening the axe.